The present invention relates to a base film for magnetic recording media which has excellent mechanical properties, surface property, running property, wear resistance and flatness.
Polyester films have excellent physical and chemical properties and are widely used in industrial fields. Above all, biaxially oriented polyethylene terephthalate films, which superior in the flatness, mechanical strength, dimension stability, etc., are now essential as a base material for magnetic recording media.
Magnetic recording media have recently been rapidly improved and with this improvement, base film is also strictly required to have improved properties.
For example, in media such as a video tape which is required to record data at high density, the base film is required to have a flatter surface. If the film does not have a flat surface, however, not only is the running property deteriorated but also the friction and wear between the film and the base material such as pin, etc. are increased, which causes various troubles such as damage of the film and production of abrasion dust. The abrasion dust causes a missing in a recording signal, namely, dropout, thereby greatly deteriorating the commercial value of the film.
It is generally known that if the surface of the film is made coarse to a certain extent, running property and the wear resistance are improved. For this purpose, a method of adding fine particles to a material polyester is adopted and partially put to practical use, but the quality of the thus-produced polyester film cannot be said to be satisfactory in these properties.
For example, when so-called deposited particles formed from a residual catalyst for producing a polyester is used, since the deposited particles are easily broken during the stretching process, the running property and the wear resistance of the film are inferior and, in addition, the film is difficult to reuse.
On the other hand, when inorganic particles such as kaolin, silicon oxide, titanium dioxide and calcium phosphate which are inert to polyesters are added to a polyester, since the particles are not broken or deformed during the stretching process and these particles provide comparatively steep protuberances, the running property is improved. However, since the particle size distribution is generally broad, the coarse protuberances frequently cause the deterioration of the electromagnetic transducing nature and dropouts.
In order to cope with this antinomy, use of inorganic or organic particles having a sharp particle size distribution has recently been proposed. For example, Japanese Patent Application Laid-Open (KOKAI) Nos. 62-207356 (1987) (EP 236945) and 59-217755 (1984) (U.S. Pat. No. 4,568,616) propose the use of monodisperse silicon oxide and the use of cross-linked organic particles obtained by emulsion polymerization, respectively. Both methods, however, have problems. In the former case, since the hardness is so high as to be apt to damage the base material which the film comes into contact with and in the latter case, the particles are easy to deform during the stretching process and inferior in the heat resistance.
As described above, in the present state of art, there in no polyester film for magnetic recording media which sufficiently satisfies the running property and the wear resistance and which also has other necessary properties.